Cholecystokinin (CCK) is a member of a group of brain/gut peptides which have distinct functions in the central nervous system and the gastrointestinal tr a ct. CCK plays an important role in the coordination of digestion and feeding behavior. CCK functions s a hormone in the gut to regulate digestion by stimulating pancreatic enzyme secretion, gall bladder contraction, and gastric motility. The specific function of CCK in the brain has not been clearly defined and has been an area of intense investigation. CCK has been localized to neurons in many different regions of the brain, where it appears to act as a neurotransmitter and neuromodulator. CCK has been demonstrated to evoke a satiety response when administered peripherally by intraperitoneal injection or centrally by intracerebroventricular injection. Furthermore, administration of specific CCK receptor antagonists has ben observed to stimulate eating. These observations suggest that CCK is a physiologic satiety factor which regulates feeding behavior. Thus, CCK may play a role in obesity or eating disorders such as anorexia nervosa or bulimia. The overriding hypothesis for this study is that CCK is a critical satiety factor necessary for the proper control of feeding behavior. In this proposal, we will use a genetic approach to further define the function of CCK. Mice deficient in CCK will be produced using the technique of gene targeting in mouse embryonic stem (ES) cells. A CCK mutation will be engineered into ES cells by replacing the endogenous gene with a mutant CCK construct containing neo and lacZ inserts disrupting the coding region (CCKlacZ). Mutant ES cells will be microinjected into normal mouse blastocysts to generate mouse strains carrying the targeted mutation. The phenotype of mice heterozygous and homozygous for the deficiency will be examined to define the essential function of CCK in regulating feeding behavior. Our hypothesis predicts that CCKlacZ mice generated in this study will also be used to characterize the spatial and temporal patterns of expression of CCK in the brain. The lacZ insert in the targeted allele will rely on the CCK promoter for expression. Thus, "CCK" transcripts will code for the lacZ gene product, beta-galactosidase. A clear definition of which specific neurons express CCK will contribute to our understanding of the phenotype on a cellular level. Gene targeting in ES cells can potentially be an important new tool to study brain/gut peptide function. The generation of CCK-deficient mice in this proposal will test the usefulness of this approach for the analysis of behavioral processes in the brain.